1. Field of the Invention
This invention relates to generation of second harmonics from lasers and more particularly to diffractive outcouplers for generation of second harmonics from lasers and for intracavity generation of second harmonics from lasers.
2. Description of Related Art
Intracavity second harmonic generation is a very efficient method for converting energy from the fundamental frequency to its second harmonic. Typically, two mirrors of approximately 100% reflectivity at the fundamental frequency are used as laser end mirrors to reach a maximum power density of radiation inside of a cavity. A nonlinear crystal, that has a high transmission at the frequency of the second harmonic, is placed inside the cavity and near one of the mirrors for second harmonic generation. The end mirrors are usually implemented using a distributed Bragg reflector that is a Bragg grating and assists in the narrowing of the laser radiation spectrum. Thus, mostly the radiation of second harmonics can leave the cavity. This method of second harmonic generation is therefore very efficient, particularly when high power and narrowband radiation from a laser are available, such as with a Nd:YAG laser oscillator. Nd:YAG lasers typically emit light with a wavelength of 1064 nm, in the infrared.
For wideband radiating lasers, such as semiconductor lasers, however, the spectrum of oscillation is wider than the acceptance bandwidth of crystal for frequency doubling. This prevents obtaining highly efficient energy conversion through the above intracavity second harmonic generation method. In particular, the wide bandwidth of a surface-emitting semiconductor laser (>1 nm) has been found to result in low conversion efficiency despite the use of a distributed Bragg reflector as one of the mirrors. High reflective thin distributed Bragg reflectors in semiconductor lasers have a rather wide band (>10 nm) and flat-topped spectral reflection. So, a surface-emitting semiconductor, even when used with distributed Bragg reflectors, cannot provide sufficiently stable and narrowband oscillation for the laser. Further, the laser tends not to operate near the phase-matching wavelength because the laser gain is highly homogeneous and the nonlinear losses are maximized for phase-matched wavelengths.
To stabilize the wavelength and bandwidth of the semiconductor laser oscillation, an etalon with a thickness of 100-200 μm may be introduced into the cavity. However, etalons have many maximums of transmission in spectral space, which are separated from each other by intervals of a few nanometers. Therefore, instead of being monochromatic, a laser including an etalon tends to oscillate by a few spectral lines when operated under the high power needed for efficient second harmonic generation. For such wide bandwidth active media, therefore, strong narrowband frequency stabilization is needed for high efficiency intracavity second harmonic generation.